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Chapter 7. Linux Containers with Docker Format

Docker is an open source project that automates the deployment of applications inside Linux Containers, and provides the capability to package an application with its runtime dependencies into a container. It provides a Docker CLI command line tool for the lifecycle management of image-based containers. Linux containers enable rapid application deployment, simpler testing, maintenance, and troubleshooting while improving security. Using Red Hat Enterprise Linux 7 with Docker allows customers to increase staff efficiency, deploy third-party applications faster, enable a more agile development environment, and manage resources more tightly.
To quickly get up-and-running with Docker Containers, refer to Get Started with Docker Containers.
Linux containers with Docker format are supported running on hosts with SELinux enabled. SELinux is not supported when the /var/lib/docker directory is located on a volume using the B-tree file system (Btrfs).

7.1. Components of Docker Containers

Docker works with the following fundamental components:
  • Container – an application sandbox. Each container is based on an image that holds necessary configuration data. When you launch a container from an image, a writable layer is added on top of this image. Every time you commit a container (using the docker commit command), a new image layer is added to store your changes.
  • Image – a static snapshot of the containers' configuration. Image is a read-only layer that is never modified, all changes are made in top-most writable layer, and can be saved only by creating a new image. Each image depends on one or more parent images.
  • Platform Image – an image that has no parent. Platform images define the runtime environment, packages and utilities necessary for containerized application to run. The platform image is read-only, so any changes are reflected in the copied images stacked on top of it. See an example of such stacking in Figure 7.1, “Image Layering Using Docker Format”.
  • Registry – a repository of images. Registries are public or private repositories that contain images available for download. Some registries allow users to upload images to make them available to others.
  • Dockerfile – a configuration file with build instructions for Docker images. Dockerfiles provide a way to automate, reuse, and share build procedures.
Image Layering Using Docker Format

Figure 7.1. Image Layering Using Docker Format